In an index table in which a rotating shaft extends in a direction intersecting a vertical direction, a center-of-gravity position of objects or members supported by the rotating shaft, that is, drive members such as a table to which a workpiece is secured, a workpiece mounting jig, and the workpiece may become considerably biased from a center of axis of the rotating shaft. For example, for a tilting index table, a rotating shaft extends in a horizontal direction, and a center-of-gravity position of a table to which a workpiece is secured is considerably biased from a center of axis of a rotating shaft. By mounting the workpiece or a workpiece mounting jig, the center-of-gravity position of drive members is moved towards the rotating shaft to slightly reduce the biasing from the center of axis. However, as the drive members are made heavier, rotary torque applied to the rotating shaft, that is, unbalance torque resulting from the biasing of the center-of-gravity position is increased. Since the unbalance torque depends upon index angle, the unbalance torque varies considerably as a result of indexing the rotation. Therefore, a driving motor of the rotating shaft requires a capacity that corresponds to load variation. In addition, a drive transmission member, such as a shaft or a coupling, and the rotating shaft need to have a rigidity that corresponds to the load variation in order to prevent index precision from being impaired by twisting, in particular, twisting degree differing with each index angle.
As a device that compensates for and reduces such an unbalance torque, an unbalance torque compensating device shown in FIG. 6 is known. The unbalance torque compensating device is applied to a tilting index table. The device shown in FIG. 6 includes a rotating shaft 91 of a table 2 to which a workpiece is secured, a first gear 93 secured to the rotating shaft 91, a second shaft 92 that supports a second gear 94 engaging the first gear 93 and having the same number of teeth as the first gear 93, a crank arm 95 having one end connected to the second gear 94 at a position that is biased from a center of axis of the second shaft 92 through a crank pin 97, and a fluid-pressure cylinder device including a piston 98 connected to the other end of the crank arm 95 through a pin 96. A direction of an urging force F of the fluid-pressure cylinder device is perpendicular to an axis of the second shaft 92 (PTL1).
As shown in FIG. 6(A), when a center of gravity G of a drive member such as a table 2 exists directly below a center of axis of the rotating table 91 and on a vertical line passing through the center of axis, an unbalance torque (a first rotary torque T1) resulting from the weight of the drive member that is supported, that is, gravity Wg (W: the mass of the drive member, g: acceleration due to gravity) is not generated at the rotating shaft 91. In addition, at this time, an urging force F of the cylinder device is oriented towards a center of axis of the second shaft 92 through the crank pin 97, and compensation torque (a second rotary torque T2) is not generated at the second gear 94. When, as shown in FIG. 6(B), the table 2 rotates counterclockwise by θ as a result of rotation of the rotating shaft 91, the second gear 94 rotates clockwise by θ, so that the crank arm 95 rotates counterclockwise by α around the pin 96 as center while the crank arm 95 moves forward. If the distance between the crank pin 97 and the pin 96 is L, α is a function of θ expressed by α=arc sin(R2×sin θ/L).
By the gravity Wg, the counterclockwise first rotary torque T1 is applied to the rotating shaft 91 as an unbalance force. In addition, if the distance from the center of axis of the rotating shaft 91 to the center of gravity G of the drive member is R1, the first rotary torque T1 is determined by T1=Wg×R1×sin θ. A force in which the urging force F acts upon the crank pin 97 through the crank arm 95 becomes F×cos α, and, at 90°-θ-α, intersects a tangent to the crank pin 97, that is, a line that is orthogonal to a line connecting the center of axis of the second shaft 92 and the crank pin 97. Therefore, by the fluid-pressure cylinder device, the second rotary torque T2 in the clockwise direction is applied to the second gear 94. In addition, if the distance from the center of axis of the second shaft 92 to the crank pin 97 is R2, the second rotary torque T2 is determined by T2=R2×F×cos α×sin(θ+α).
The clockwise second torque T2 acting on the second gear 94 is transmitted to the rotating shaft 91 through the first gear 93 engaging the second gear 94, and acts as a counterclockwise rotary torque with respect to the rotating shaft 91, so that it acts as a compensation torque acting in a direction opposite to the unbalance torque. Therefore, an absolute value of the following rotary torque (obtained by subtracting the second rotary torque T2, applied by the fluid-pressure cylinder device, that is, the compensation torque, from the clockwise first rotary torque T1, applied by the gravity Wg, that is, the unbalance torque) is applied to the rotating shaft 91 as a clockwise or a counterclockwise correction torque, so that a load of, for example, a driving motor of the rotating shaft 91 is reduced.T1−T2=Wg×R1×sin θ−R2×F×cos α×sin(θ+α)
As mentioned above, α is a function of θ, and the correction torque T1-T2 is a function of θ, so that it changes along with a rotational angle θ of the table 2. Therefore, in order to minimize the correction torque T1-T2, and to maintain it at a certain value, it is necessary to change the urging force F of the fluid-pressure cylinder device as the table 2 rotates, so that control of the urging force F at the fluid-pressure cylinder device is required.
An unbalance torque compensating device differing from the above-described device is known. In this device, instead of connecting the fluid-pressure cylinder device to the second gear and applying an urging force thereto, a balance weight is mounted to the second gear, to compensate for an unbalance torque. In the device, the balance weight is mounted to the second gear at a location that is biased from a center of axis, and is provided with a space for rotation thereof for preventing interference with other devices and injury to an operator (PTL 2).